Fiber light sources (FLS) are commonly used with interferometric fiber optic gyroscopes (IFOGs) because of their broad bandwidth, high power, and relatively good mean wavelength stability compared to other compact light sources. With the development of reliable pump lasers and improved output power and wavelength control, the reliability and stability of such fiber light sources have improved significantly since their inception. However, there are still demands to improve FLS reliability and mean wavelength stability in high end IFOGs in mission-critical applications.
The reliability of the FLS depends in large part on the reliability of the pump laser. Therefore, use of redundant or backup pumps can substantially increase the FLS reliability. In one configuration to accommodate a backup pump, a 2×2 fiber coupler is used to combine and direct the outputs of two pump lasers to the gain medium. This configuration results in waste of half of the pump power when a single end pumping scheme is used. In another configuration, dual-end pumping schemes are used to improve the efficiency, however, these configurations result in compromised wavelength stability due to sensitivity to power splitting ratio changes of the 2×2 coupler.
FLS mean wavelength instability on the order of a few tens of parts per million (ppm) is common even after tight control of pump laser power and pump wavelength fluctuations. This FLS wavelength instability originates from polarization hole burning effects in Erbium Doped Fiber (EDF) due to instability of pump laser polarization states in the single mode EDF. Depolarizing the pump light by fiber depolarizers and use of polarization maintaining (PM) EDF have been suggested in prior art to reduce this effect. Polarization multiplexing of two pump lasers with equal power and orthogonal polarization states was suggested. However, this scheme requires simultaneous operation of two pump lasers with strict requirements in polarization state and power in order to reduce the mean wavelength instability. If one pump is used as a backup in this scheme, the only pump that is turned on will not be depolarized, leading to FLS wavelength instability due to polarization hole burning in EDF.